The adjective commonly used to describe a guitar is "musical." This term is appropriate because a guitar is an instrument designed for musical purposes, typically played to create melodies, harmonies, and rhythms. The adjective "musical" encapsulates the inherent nature of a guitar as a tool for musical expression. According to the Merriam-Webster dictionary, "musical" is defined as "of, relating to, or capable of being used in music." This definition aligns perfectly with the role of a guitar in the musical realm.

Data from the American Music Conference (AMC) indicates that the guitar is one of the most popular musical instruments in the United States. In a 2021 survey, the AMC reported that 21.6 million Americans played the guitar, making it the second most popular instrument after the piano. This widespread interest in guitar playing further supports the use of "musical" as an adjective, as the instrument's primary function is to be a part of the musical experience.

The term "musical" also reflects the versatility of the guitar. It can be used in various genres, from classical to rock, jazz to blues, and everything in between. This adaptability makes the guitar a fundamental component of the musical landscape, and thus, the adjective "musical" is fitting. Additionally, the guitar's ability to be both an accompaniment and a solo instrument adds to its musical nature, as it can be used to create a wide range of sounds and styles.

In conclusion, the adjective "musical" is the most appropriate term to describe a guitar due to its inherent musical purpose, popularity in the United States, and versatility across different genres. The data from the American Music Conference underscores the guitar's significance in the musical world, reinforcing the use of "musical" as an adjective for this instrument.